Cold-workable corrosion-resistant chromium steel

ABSTRACT

A cold-workable, corrosion-resistant ferritic chromium steel, comprising 0,005 to 0.1% of carbon, 0.2 to 1.2% of silicon, 0,4 to 2.0% of manganese, 8 to 20% of chromium, 0.1 to 1.2% of molybdenum, 0.01 to 0.5% of nickel, 0.5 to 2.0% of copper, 0.001 to 0.6% of bismuth, 0.002 to 0.01% of vanadium, 0.002 to 0.1% of titanium, 0.002 to 0.1% of niobium, 0.15 to 0.8% of sulfur and 0.001 to 0.08% of sulfur and 0.001 to 0.08% of nitrogen, remainder iron including smelting-related impurities, is suitable as a material for precision-mechanics applications and precision appliances, in particular spinning and spraying nozzles, tips and heads for writing implements, on account of its good mechanical processibility, in particular its good machinability, its good wear resistance and surface quality.

[0001] The invention relates to a cold-workable, corrosion-resistant chromium steel, in particular with a ferritic microstructure.

[0002] Steels of this type are known. They have a good magnetizability, such as for example the soft-magnetic steel described in U.S. Pat. No. 4,714,502, which comprises up to 0.03% of carbon, 0.40 to 1.10% of silicon, up to 0.50% of manganese, 9.0 to 19% of chromium, up to 2.5% of molybdenum, up to 0.5% of nickel, up to 0.5% of copper, 0.02 to 0.25% of titanium, 0.010 to 0.030% of sulfur, up to 0.03% of nitrogen, 0.31 to 0.60% of aluminum, 0.10 to 0.30% of lead and 0.02 to 0.10% of zirconium. The steel is stainless and cold-workable. It is suitable as a material for the production of cores for solenoid valves, electromagnetic couplings or housings of electronic injection systems for internal combustion engines.

[0003] A further soft-magnetic stainless chromium steel comprising up to 0.05% of carbon, up to 6% of silicon, 11 to 20% of chromium, up to 5% of aluminum, 0.03 to 0.40% of lead, 0.001 to 0.009% of calcium and 0.01 to 0.30% of tellurium is known from U.S. Pat. No. 3,925,063 and has a good machinability on account of its lead, calcium and tellurium contents. However, a drawback of this steel is the use of the toxic elements lead and tellurium, which improve the machinability.

[0004] In this steel, however, the relatively high levels of silicon, aluminum and titanium, on account of the formation of hard oxide inclusions, lead to a high level of wear during precision machining. This is intended to be counteracted by the relatively high lead content of 0.03 to 0.40%. However, the cost of this is a not inconsiderable danger to environment and health on account of the toxic lead.

[0005] Finally, U.S. Pat. No. 5,190,722 has disclosed a further cold-workable stainless steel comprising up to 0.02% of carbon, up to 0.5% of silicon, up to 0.5% of manganese, 10 to 18% of chromium, 0.3 to 1.50% of molybdenum, up to 1.0% of vanadium, 0.05 to 0.5% of titanium, up to 1.0% of niobium, 0.01 to 0.2% of sulfur, up to 0.05% of nitrogen, 0.30 to 2.0% of aluminum and 0.0005 to 0.05% of boron. This steel is suitable as a material for valve housings and valve cores in electronically controlled fuel injection systems. In this steel too, the high levels of aluminum and titanium lead to hard, unevenly distributed oxide precipitations, which have an adverse effect on the mechanical processibility, in particular on the chip-forming machinability.

[0006] A common characteristic of many cold-workable corrosion-resistant ferritic chromium steels is their poor machinability on account of material sticking in the region of the cutting edge. This sticking material comprises generally oxidic welded-on or deposited material which leads to considerable wear to the sharp cutting edges of the machining tools or even to the edges fracturing. This risk is particularly high in the case of miniaturized precision components and the micromachining thereof. For example, in the case of microdrilling in the diameter range from 0.2 to 1 mm, considerable tool wear occurs at the particularly sharp-edged drill bits. Moreover, as the diameter of the drill or drilled hole increases, the risk of lateral drift of the drilled hole or of a loss of straightness in the drilled hole increases. Moreover, a burr is normally formed at the edges of the drilled hole, and this burr becomes more pronounced as the machinability deteriorates. There are similar problems with the chip-forming production of grooves, recesses, blind holes and slots.

[0007] The cause of the abovementioned drift is inhomogeneities in the microstructure, in particular hard precipitations in the form of nests and islands of titanium carbides, titanium carbonitrides, titanium nitrides, manganese sulfide and heterogeneous silicon—aluminum oxides. The precipitations cause thin microdrills, for example with diameters of below 0.5 mm, and slender microtools to deviate toward softer material zones. Of course, deviation of this type does not occur if the precipitations are more finely dispersed or are more homogeneously fine-grained and are distributed throughout the microstructure.

[0008] Hitherto, with conventional ferritic steels, it has been aimed to improve their formability or cold-workability with the aid of alloying elements. However, the alloying elements which have a favorable effect on the formability often entail a deterioration in the machinability, which can explain the poor machinability of ferritic steels with good cold-workablility. One characteristic of poor machinability is wear to the tool cutting edge. This wear occurs as abrasion, flank wear, lime wear, diffusion wear, oxidation wear, or built-up edges and stuck material are formed in particular during the machining of ferritic steels with a low carbon content.

[0009] Working on the basis of this prior art, it is an object of the invention to provide a cold-workable, corrosion-resistant chromium steel with improved machinability, in particular with a low tendency to form built-up edges and/or stuck material, which in particular allows directionally accurate drilling, punching and stamping even if tools with a small cross section and low rigidity, for example microdrills are used.

[0010] To achieve this object, the invention proposes a steel comprising at least 8% of chromium and at most 0.1% of carbon as well as specific levels of manganese and/or bismuth, titanium and/or vanadium and/or niobium and sulfur and copper, which in the melt lead to primary precipitations, in the form of sulfocarbides of the metals titanium, vanadium and niobium of type Me₄C₂S₂, for example Ti₄C₂S₂. The sulfocarbides are finely distributed in the melt and serve as nuclei for manganese sulfide precipitations, which are then distributed correspondingly uniformly and finely in the melt. The presence of bismuth promotes the finely dispersed and homogeneous distribution of the manganese sulfide in the steel.

[0011] Copper has a similar effect, apparently improving the wettability of the manganese sulfide and in particular changing its wetting angle with respect to the iron/chromium matrix in such a way that finely dispersed, spherical, cigar-shaped and constricted manganese sulfate precipitations are formed.

[0012] Bismuth promotes the precipitation of the titanium sulfocarbides and in this way brings about a finely dispersed precipitation of the manganese sulfide even if the melt is slightly supersaturated.

[0013] The effect of the alloying elements which promote machinability, for example of bismuth and copper, is synergistic.

[0014] To suppress the formation of titanium carbide and to promote the formation of finely dispersed sulfocarbides, the levels of the machining-enhancing elements titanium, vanadium, niobium, on the one hand, and of the carbon and sulfur, which are responsible for the formation of sulfocarbides on the other hand, should be matched to one another in a specific way. Then, it is no longer necessary to have higher levels of manganese and sulfur to improve the micromachinability, since the manganese sulfide is present in larger cohesive agglomerates. At the same time, the formation of intermetallic titanium/aluminum precipitations, which impair machinability, is suppressed, preventing titanium and aluminum from being dissolved, thus increasing the tendency to form sticking material and built-up edges.

[0015] The nitrogen content of the steel should be as low as possible, in order not to impair the formation of primary nuclei comprising titanium carbosulfides as a result of the titanium bonded in the form of TiN.

[0016] In detail, the chromium steel according to the invention contains

[0017] 0.005 to 0.1% of carbon

[0018] 0.2 to 1.2% of silicon

[0019] 0.4 to 2.0% of manganese

[0020] 8 to 20% of chromium

[0021] 0.05 to 1.2% of molybdenum

[0022] 0.01 to 0.5% of nickel

[0023] and, in detail in combination with one another

[0024] 0.5 to 2.0% of copper

[0025] 0.001 to 0.6% of bismuth

[0026] 0.002 to 0.10% of vanadium

[0027] 0.002 to 0.10% of titanium

[0028] 0.002 to 0.10% of niobium

[0029] 0.15 to 0.80% of sulfur

[0030] up to 0.05% of aluminum

[0031] up to 0.08% of nitrogen,

[0032] remainder iron

[0033] It is preferable for the chromium steel according to the invention, in each case within the limits stipulated above, to contain:

[0034] 0.002 to 0.06% of carbon

[0035] 0.3 to 0.8% of silicon

[0036] 0.5 to 1.6% of manganese

[0037] 11 to 18% of chromium

[0038] 0.05 to 0.8% of molybdenum

[0039] 0.01 to 0.1% of nickel

[0040] 0.55 to 1.60% of copper

[0041] 0.002 to 0.22% of bismuth

[0042] 0.005 to 0.08% of vanadium

[0043] 0.005 to 0.08% of titanium

[0044] 0.005 to 0.08% of niobium

[0045] 0.15 to 0.65% of sulfur,

[0046] remainder iron.

[0047] To promote the formation of sulfocarbides in a finely dispersed and homogenous solution, the alloying elements titanium, vanadium and niobium or sulfur, carbon and nitrogen or copper and manganese should be matched to one another in the following ways: $\begin{matrix} {{K1} = {{\% \quad T\quad i} + {\% V} + {\% N\quad b}}} \\ {{{K1} = {0.005\quad t\quad o\quad 0.15}}} \\ {{K2} = \frac{\% S}{10*\left( {{\% C} + {\% N}} \right)}} \\ {{{K2} = {0.8\quad t\quad o\quad 3.8}}} \\ {{K3} = \frac{\% C\quad u}{{\% C\quad u} + {M\quad n}}} \\ {{K3} = {0.25\quad t\quad o\quad 0.85}} \end{matrix}$

[0048] On account of its good machinability, the chromium steel according to the invention is suitable as a material for the production of precision appliances and highly accurate microcomponents with low tool wear with microbores and recesses, for example in the region of tenths or hundredths of a millimeter, a high surface quality and directional accuracy. By way of example, it is possible to produce drilled holes with a diameter of below 1 mm without any drift in a single operation. Furthermore, the steel has excellent polishing properties, in particular under electropolishing.

[0049] It is particularly advantageous that the improved machinability results without high contents of toxic alloying constituents, such as lead, selenium and/or tellurium, which are absent altogether or the total amount of which is below 0.05%.

[0050] The chromium steel according to the invention is suitable, for example, as a material for writing tips of ballpoint pens. Writing tips of this type and the associated writing heads require a high resistance to corrosion, precision-machinability and uniformity of the ink supply. The front part of the writing head of a ballpoint pen comprises a holder for the writing ball, for example made from corundum, and a plurality of passages and bores for supplying the ink. The rear part of the writing head generally comprises a connection to a reservoir, for example a metal or plastic cylinder for the ink, which may also be under pressure. The supply of the ink to the writing ball is effected via a central precision-bored passage with a diameter of less than 0.5 mm and a plurality of laterally and symmetrically arranged recesses. The central precision-bored passage must be positioned in such a way that the writing ball and the recesses which are arranged symmetrically with respect thereto are met precisely centrally by the ball, since only then will the writing ball be wetted uniformly on all sides with ink as it rotates. If these conditions are not satisfied, for example as a result of lateral drift of a drilled hole, the writing ball will correspondingly only be covered with ink on one side. When it is being used to write, this then leads to an uneven character strength and a poor appearance of the writing.

[0051] A further condition for a uniform supply of ink to the writing ball is a high resistance to corrosion and surface quality, which manifests itself in a corresponding brightness and reflection, and a good wettability.

EXAMPLE 1

[0052] To produce injection nozzles for a plastic monofilament, first of all a wire with a diameter of 3 mm and the composition listed under V1 in Table I with the following K values:

[0053] K1=0.08

[0054] K2=1.94

[0055] K3=0.59

[0056] and a length of 4.4 mm was straightened. Then, the wire was cut into disks and the disks were shaped in a press at room temperature, with a degree of deformation of φ=0.45 to form nozzle blanks with a disk thickness of 2.8 mm. Then, the nozzle blanks are centrally drilled open in an automated drilling machine using a sintered-carbide drill bit with a diameter of 0.4 mm. Only a very small burr was formed during drilling, and electropolishing for twenty seconds with simultaneous rounding of the edge of the drilled hole was easily able to remove this burr, leaving behind a bright surface.

[0057] After cleaning and drying, the nozzles were ready for use.

[0058] The quality of the nozzle surface, which is excellent on account of the good machinability, results in a low wall friction and allows spinning with a relatively low delivery pressure even when using molten plastics with a high viscosity.

EXAMPLE 2

[0059] In a similar manner to that described in Example 1, a chromium steel wire with a composition as listed under V6 in Table I and having the following K-values:

[0060] K1=0.13

[0061] K2=1.26

[0062] K3=0.47

[0063] was cold-formed into a nozzle blank with a disk thickness of 5.5 mm and a sealing press fit. To produce a nozzle opening with a diameter of 85 μm, the blank was provided with six blind bores, each with a diameter of 0.8 mm and a depth of 4.9 mm, in an automated drilling machine. After cleaning in an ultrasound bath and drying with hot air, bores with the predetermined diameter of 85 μm were melted into the base of the blind bores with the aid of an Nd YAG laser. Laser-drilling of this type causes problems if the blind bore does not run in a straight line. Such problems did not occur in the test. Moreover, the absence of lead, selenium and tellurium meant that there were no toxic metal vapors in the test steel. The precision of the drilled hole produced in this way allows further processing to form straight, curved or even star-shaped slots.

EXAMPLE 3

[0064] To assess the machinability of the chromium steel according to the invention and the straightness of microbores drilling tests were carried out using sintered-carbide drill bits in a diameter range from 0.2 to 1.5 mm, in particular with a drill bit diameter of 0.8 mm and a rotational speed of 37,000 rpm and a drilled-hole depth L of 5 mm in each case were carried out.

[0065] The straightness of the drilled holes was determined for each drilled-hole diameter using a test wire, the diameter of which was approximately 10 μm smaller than the drilled-hole diameter and the penetration depth E of which was determined in accordance with the illustration shown in FIG. 1. A curvature factor, which at KR=0 indicates a completely straight or drift-free drilled hole, was in each case calculated from the penetration depth E and the drilled-hole depth L in accordance with the following formula

KR=1−E/L.

[0066] The analyses of the test steels V1 to V6 according to the invention and of comparison steels V7 to V12 and the measurement results are compiled in the following Tables I and II. TABLE I Alloy C % Si % Mn % P % S % Cr % Ni % Mo % Al % N % V % Ti % Nb % Cu % Bi % Pb, Se, Te V1  0.008 0.63 0.71 0.025 0.31 17.34 0.24 0.21 0.003 0.008 0.06 0.01 0.005 1.03 0.002 n.d V2  0.006 0.72 0.86 0.03 0.33 17.56 0.08 0.32 0.002 0.006 0.04 0.01 0.008 1.15 0.005 n.d V3  0.006 0.65 1.05 0.02 0.52 17.60 0.10 0.23 0.002 0.008 0.05 0.02 0.01 0.86 0.01 n.d V4  0.015 0.42 0.75 0.02 0.26 17.20 0.25 0.06 0.002 0.006 0.02 0.08 0.01 1.05 0.005 n.d V5  0.020 0.45 0.78 0.01 0.25 12.40 0.18 0.15 0.002 0.010 0.03 0.03 0.01 1.25 0.01 n.d V6  0.035 0.50 1.12 0.03 0.54 11.50 0.10 0.10 0.004 0.008 0.01 0.10 0.02 0.98 0.20 n.d V7  0.006 0.82 1.48 0.02 0.35 17.05 0.12 0.45 0.003 0.005 0.003 0.001 0.001 0.01 <0.001 n.d V8  0.015 0.45 0.42 0.02 0.03 15.20 0.10 0.08 0.002 0.008 0.002 0.30 0.002 0.02 <0.001 n.d V9  0.015 0.65 0.52 0.03 0.004 18.00 0.15 0.02 0.003 0.015 0.005 0.35 0.002 0.04 <0.001 n.d V10 0.012 0.55 0.85 0.02 0.03 14.60 0.15 0.05 0.003 0.010 0.02 0.22 0.01 0.23 0.08 n.d V11 0.090 0.32 0.38 0.01 0.002 12.45 0.15 0.05 0.002 0.028 0.001 0.008 0.001 0.01 <0.001 n.d V12 0.012 0.48 1.760 0.030 0.250 20.11 0.250 1.840 0.003 0.010 0.001 0.005 0.029 0.02 <0.001 Pb: 0.12 Se: 0.018 Te: 0.005

[0067] TABLE II Micromachining Alloy K1 K2 K3 KR = 1 − E/L BG/mm Suitability V1  0.08 1.938 0.59 1 0.060 very good V2  0.06 2.750 0.57 1 0.058 very good V3  0.08 3.714 0.45 1.00 0.072 good V4  0.11 1.238 0.58 1 0.065 very good V5  0.07 0.833 0.62 1 0.035 very good V6  0.13 1.256 0.47 1 0.058 very good V7  0.01 3.182 0.01 1.00 0.201 poor V8  0.30 0.130 0.05 1.00 0.193 poor V9  0.36 0.012 0.07 1.00 0.212 poor V10 0.25 0.136 0.21 1.00 0.205 poor V11 0.01 0.002 0.03 1.00 0.187 poor V12 0.035 1.136 0.01 0 0.059 very good V12 contain toxic elements Key: Desired value for K1 Desired value for K2 Desired value KR BG 0.005.0.15 0.8..3.8 0.25.0.85 “Curvature” Drilling burr in mm

EXAMPLE 3

[0068] During the production of microbores with diameters of below 1 mm, the formation of the chip is of considerable importance to the drill wear and to the quality of the drilled hole. Insufficient chip formation and the suitability of a material for the production of microbores can easily be derived from the height or width of a drilled burr. A wide drilled burr is an indication of poor machinability, since the material is then squeezed out of the drilled hole and a burr is formed at the side or edge of the drilled hole.

[0069] The width of the burrs was measured in a series of tests with the aid of a microscope at an angle of 20 to 30 degrees. The above Table II lists the burr width GB as a function of the K factors, while FIGS. 2 and 3 show electron microscope images of microbores with different widths of drilled burrs BG. FIG. 2 clearly shows the sudden improvement in the suitability of the burr in the test carried out with the steel according to the invention, with a burr width of only 0.060 mm, compared to a burr width of 0.187 mm in the comparison steel in accordance with FIG. 3. 

1. A chromium steel, comprising 0.005 to 0.1% of carbon 0.2 to 1.2% of silicon 0.4 to 2.0% of manganese 0.05 to 1.2% of molybdenum 0.01 to 0.5% of nickel and, individually or in combination with one another, 0.5 to 2.0% of copper 0.001 to 0.6% of bismuth 0.002 to 0.10% of vanadium 0.002 to 0.10% of titanium 0.002 to 0.10% of niobium 0.15 to 0.80% of sulfur up to 0.05% of aluminum up to 0.08% of nitrogen, remainder iron
 2. The chromium steel as claimed in claim 1, comprising 0.002 to 0.06% of carbon 0.3 to 0.8% of silicon 0.5 to 1.6% of manganese 11 to 18% of chromium 0.05 to 0.8% of molybdenum 0.01 to 0.1% of nickel and, individually or in combination with one another, 0.55 to 1.60% of copper 0.002 to 0.22% of bismuth 0.005 to 0.08% of vanadium 0.005 to 0.08% of titanium 0.005 to 0.08% of niobium 0.15 to 0.65% of sulfur, remainder iron.
 3. The chromium steel as claimed in claim 1 or 2, characterized in that it satisfies at least one of the following three conditions: $\begin{matrix} {{K1} = {{\% \quad T\quad i} + {\% V} + {\% N\quad b}}} \\ {{{K1} = {0.005\quad t\quad o\quad 0.15}}} \\ {{K2} = \frac{\% S}{10*\left( {{\% C} + {\% N}} \right)}} \\ {{{K2} = {0.8\quad t\quad o\quad 3.8}}} \\ {{K3} = \frac{\% C\quad u}{{\% C\quad u} + {M\quad n}}} \\ {{K3} = {0.25\quad t\quad o\quad 0.85}} \end{matrix}$


4. The use of the steel as claimed in one of claims 1 to 3 for the production of objects by machining.
 5. The use of the steel as claimed in one of claims 1 to 3 for the production of objects by micromachining.
 6. The use of the steel as claimed in one of claims 1 to 3 as material for the production of precision appliances, microcomponents, pen tips and heads, printer nozzles, metering devices and electronic components with openings and recesses of ultra small dimensions. 